Image forming apparatuses such as copying machines or printers that use an optical shutter array are known. Such known optical shutter arrays comprise optical shutter elements that are micro-pixel units having an electrooptical Kerr effect, and are used as a printer head that exposes a photoreceptor. Typically, the optical shutter element comprises PLZT, or some other material having a large Kerr constant. In operation, the optical shutter elements either shield the light from the light source or allow the light to pass so as to form an image on the surface of the photoreceptor. An image is then formed on the recording sheet based on the image formed on the photoreceptor. Notwithstanding the widespread use of optical shutter arrays, as explained below known arrays suffer various problems resulting from heat/power dissipation.
For example, in a typical optical shutter array having a recording density of approximately 400 dpi, the capacitance of each element (optical shutter element) of the optical shutter array is approximately 20 pF. Thus, if the drive voltage for the optical shutter array is 50 V and the drive frequency is 2 kHz, the power consumption for an element when one element is driven is 0.1 mW pursuant to formula (G) set forth below.
Continuing this example, if the drive frequency for a 400 dpi image forming apparatus is set at 2 kHz, the recording speed of the image recording device will be approximately 10 cm/sec. For this image forming apparatus to handle high-speed printing at speeds of up to 100 cm/sec, the drive frequency must be raised to 20 kHz. When the drive frequency is 20 kHz, however, the power consumption per element is 1 mW.
Further, when the recording density is 400 dpi, 4,725 elements are needed to form an A3-size print head. If such a print head is driven with a 20 kHz drive frequency, when all of the elements are operating, the power consumed by the entire optical shutter array is 1 mW.times.4,725 =4.725W. This power is consumed mainly by the drive ICs that perform switch driving regarding the charging and discharging of the optical shutter array, and is released as Joule heat. The drive ICs are ordinarily mounted near the optical shutter array. However, the optical shutter elements of the optical shutter array are affected by the heat released in the foregoing manner due to their temperature characteristic.
FIG. 8 is a drawing showing the temperature characteristic with regard to the optimal drive voltage for the optical shutter array. Herein, the optimal drive voltage means the drive voltage that maximizes the amount of light that passes through the optical shutter array.
Referring to FIG. 8, it is shown that when the temperature of the optical shutter array increases, the optimal drive voltage increases. If the drive voltage for the optical shutter array is increased in order to reach the optimal drive voltage level, the power consumed by the drive ICs increases, based on formula (G) explained below, and the temperature of the optical shutter array increases further. When the temperature of the optical shutter array increases, the optimal drive voltage also increases. The drive voltage for the optical shutter array must then be increased further to reach the optimal drive voltage level, thereby creating a vicious cycle.
In addition, regardless of this vicious cycle, the heat generated by the drive ICs may damage the ICs. While it is possible to reduce the increase in the temperature of the drive ICs by cooling them via air cooling, etc., since the drive ICs are ordinarily sealed inside the print head and are located in a linear fashion along the optical shutter array, it is difficult to effectively cool the optical shutter array.
Accordingly, the object of the present invention is to resolve the foregoing problems. More specifically, it is an object of the present invention to provide an optical shutter element drive mechanism that can prevent the temperature increase of the switching elements used for the driving the optical shutter elements in a simple, cost efficient manner.